Secret telephony



May 16, 1939. P. KOTOWSKI ET AL 2,158,562

SECRET TELEPHONY Filed June 15, 1936 2 Sheets-Sheet 2 ADD/T/ON AAAAAA rvvvvv INVENTORS PAUL KOTOWSKI AND KURT. DANNEHL /f 5) J W v ATTORNEY Patented May 16, 1939 ,isatz SECRET TELEPHONY Paul Kotowski and Kurt Dannehl, Berlin, Germany, assignors to Telefunken Gesellschaft fiir Drahtlose Telegraphie m. b. H., Berlin, Germany, a corporation of Germany Application June 13, 1936, Serial No. 85,006 In Germany June 17, 1935 6 Claims.

The present invention relates to a method by means of which it is possible greatly to reduce the residual disturbance potential in the secret telephony according to the disturbance principle.

5 In the method of secret telephony operating on the principle of superposing a disturbance voltage on the speech voltage as known in the prior art, it is found to be of a disadvantage that the deciphered speech is still greatly distorted by H residual disturbances, since when using ordinary amplifiers and receivers a maximum ratio of disturbance reduction of only 1:8 can be attained without the faults in the high-frequency transmission i. e. at direct transmission; the residual disturbance voltage therefore is still th of the original one.

The invention is based upon the teaching that this residual voltage instead of being caused by unsymmetrical conditions in the disturbance 1:) generators, is due to elements in the transmission and receiving amplifiers, depending upon their phase, frequency, and amplitude. A closer study reveals that for instance a 20 phase displacement in the ends of the transmission range is g; sufiicient to cause such a low factor of disturbance reduction.

According to the invention, all respective parts in the transmitter and receiver from those in the disturbance generator up to the ciphering and 5.) deciphering stage are arranged entirely symmetrical with respect 'to the coupling members. In order to obtain a more distinctly deciphered speech, it is essential that the phase displacements within the entire audible range and inherent to the superposition, deviate from each other up to a value of only 5, which condition is to be considered starting from the terminals of the disturbance generator in the transmitter up to the terminals of the disturbance generator in as the receiver. These phase displacements are suitably avoided by using exclusively resistancecapacity coupling. By suitable choice of the condensers and resistors, the range of the lowfrequency transmission will always be consider- 45 ably wider than the range to be reproduced in the audible region.

This invention will best be understood by referring to the accompanying drawings, in which:

Fig. l is a circuit diagram of a symmetrical arrangement of both a transmitter and a receiver;

Fig. 2 indicates a circuit diagram of a dynamical speech control portion of a transmitter;

Fig. 3 is a circuit diagram of a portion of a transmitter indicating how the speech potential 5.; is added to the disturbance potential;

Fig. 4 is a circuit diagram of a separate excitation and grid potential modulation portion of a transmitter; and

Fig. 5 is a circuit diagram of a complete receiver which is symmetrical with the transmitter shown by Figs. 2, 3 and 4.

Fig. 1 shows by way of example such circuit with ordinary triodes, and in which a ratio of disturbance reduction up to 1:25 can be attained.

The speech voltage is transmitted across an input transfomer I, from the microphone M1 to the grid of tube V1, and this is simultaneously fed with the disturbance voltage transmitted by the disturbance generater ST1. In taking care that the apparent resistances of each of the two elements are low as compared with the gridcathode resistanec of tube V1 such as for instance 1% or approximately 10,000 ohms of the latter, the transmission of the two voltages to the grid is obtained in true amplitudes and shape. In the plate circuit of the tube, the plate is coupled to the output of a series condenser 2, a telephone T1 is inserted by means of which the disturbed speech voltages can be heard. Telephone T1 is shunted by a resistance 3. Now, from output points A and B the disturbed speech voltage is transmitted on lines or by wireless to the input points C and D at the plate side of the deciphering tube V2, whose circuit is to be arranged with coupling condenser 2A, resistance 3A and other associated circuit elements exactly the same as that of the ciphering tube V1. Assuming an imaginary symmetry plane shown by dash line 4 between the points A, B and C, D, then the circuit with its elements shown at the right side of said plane appears as the mirror image of the circuit and its elements at the left side. In the grid circuit of tube V2, the disturbance generator 8T2 produces the disturbance potential, namely of opposite phase and contained in the arriving energy, thus receiving in the telephone T2 the deciphered speech. If in both cases, the tubes operate on the straight part of the tube characteristic curve, a pure addition of disturbanceand speech potential is obtained in the transmitted mixture. Such working produces as a rule the clearest transmission. In View of the easier covering up of the speech with a given disturbance potential, operation in the curved part of the tube characteristic curve and hence a modulation of the speech by the disturbance may often be desirable. When using multi-grid tubes, it is advisable to connect the disturbance generator and the microphone to different grids. If for instance for the production of the disturbance noise generators comprising rotating disks with irregular teeth and magnetic sound pickup are employed, these parts are to be constructed identically for both the transmitter and receiver, that is, in so far as the mechanical and magnetic properties are concerned. With separately excited sound pickups the disturbance potential must not be controlled by varying the exciter current, since owing to the displacement of the preliminary magnetizing a corresponding variation in the curve shape of the disturbance potential is thereby produced. In this case,'it is no longer possible to cover up and to extinguish the disturbance potential received, so that the ratio of the disturbance reduction will be considerably impaired. An amplitudinal control of the disturbance potential is permissible only across high-ohmic voltage dividers.

In case of wireless transmission equal amplitudes of the disturbance potential can be realized only by means of a receiver with automatic control as long as the reception is not entirely free of fading. In view of the various fading intervals occurring very rapidly in many cases, an adjustment by hand is not possible where it is aimed at a useful reduction of disturbance of the transmission. The high-frequency amplifier stages of the receiver controlled for instance, are required fully to compensate for variations in the transmitter field strength at the place of reception, so that there prevails at the detector output always a potential having a constancy of 5 to 10% in accordance with the modulation of the transmitter. The demodulator is to comply with linearity, since otherwise the disturbance potentials of the disturbance generators at the transmitter-and receiver side are not a mirror image of each other, thus making a disturbance reduction impossible.

The residual disturbance potential in the telephone of the listener is the smaller the less it exceeds at the transmitter side the maximum speech potential i. e. the smaller the covering up ratio between speech and disturbance. In order that incidental speech voltage peaks be still efiectively covered by a low disturbance potential, the amplitude of the speech potential will be controlled before being added to the disturbance potential, or before being modulated therewith. Thus, the disturbance potential can be fixedly established for a station, and even very loud words can still be effectively covered up. A volume control can be provided for instance in that the alternating microphone potential is applied separately across two amplifier tubes,

whereby the one serves as feedback blocking tube with but small amplification, while the second tube furnishes a grid biasing potential for an exponential tube in accordance with the alternating microphone potential such that at large speech voltage amplitudes, the amplification factor of the exponential tube decreases. In attaining the lowest disturbance potential the useful speech modulation of the transmitter increases at the same time, so that a favorable range or size of the transmitter can be provided.

By means of a reduction of the dynamical distortion the residual disturbance noise as compared with the speech can be still further reduced at the receiver side. This can be accomplished for instance, by using a circuit as before described, in which the detector for the grid biasing potential of the exponential tube is connected in reverse. The exponential tube is biased to a high negative value, and the detector supplies a counter potential according to amplitude, so that for small amplitudes the degree of amplification of the arrangement is small, while increasing with an increase of the amplitudes. The small amplitudes of the residual disturbance potential are thus but slightly amplified, and the largest speech amplitudes are enhanced still further.

Fig. 2 shows by way of example a dynamical speech control. The alternating microphone potential is applied by a transformer 6 to the grid circuits of tubes V1 and V2. A transformer l is inserted in the plate circuit of tube V2 and which passes the amplified alternating potential to the detector GI. From resistor R and across a filter chain including condenser 8 and resistance ll, a detected potential corresponding to the speech amplitude, is applied to condenser C as grid biasing potential for tube V3. The pole arrangernent of the detector is such that with increasing amplitude of the speech potential the negative bias for tube V3 increases. V3 is a tube having exponential characteristic, so that the amplification can be controlled within wide limits by displacing the biasing potential. Tube V1 takes care of the transmission of the alternating microphone potential to tube V3 by means of transformer I. It is necessary to interpose tube V1 to avoid reaction of the detected potential upon the transformer of the microphone Hi. There can be derived from the plate circuit of V3 a voltage only slightly influenced by the speech volume.

Fig. 3 shows for instance the manner in which the speech potential is added to the disturbance potential. In order to maintain the phase error as small as possible, there is chosen a suitably dimensioned resistance H and condenser 12 for a coupling device. The exciter circuit at the 'sound pickup of the disturbance generator ST contains a variable resistor VR which however can be used only for the voltage compensation of battery B. For the purpose of avoiding phase errors the mixture is applied from the plate circuit of the tube V4 across a resistance I3, condenser C1 forming a coupling device to the transmitter as modulation potential, whereby accord ing to the output of the transmitter an additional preliminary amplifier is to be interposed.

The transmitter is adapted for separate excita tion and grid potential modulation as shown by Way of example in Fig. 4. Tube V1 is a returncoupled oscillation generator from whose plate circuit the high-frequency potential is transmitted across the capacitive voltage divider comprising condensers C1, C2 to the grid circuit of tube V2. The high-frequency choke L blocks the low-frequency paths against high-frequency. The modulation potential is applied at resistance R. Attention is to be paid to an eventual phase error when dimensioning C1, 02, L and R, but such error may be readily avoided also in this case. In the circuit shown, V2 is at the same time the power tube, and takes care of delivering the modulated high-frequency to the antenna across a suitable matching means.

Fig. 5 shows by way of example a mode of operation of the receiver. The modulated highfrequency is applied to the linear detector across an automatically controlled high-frequency amplifier. There is applied to the mixture of speech and disturbance, that can be derived from the detector output, the disturbance potential of the local generator in opposite phase (stage of addition) thereby removing the disturbance potential save the residual part. This speech which is already quite pure and intelligible, is passed across the combination comprising tubes V1, V2, V3 to the telephone F thereby increasing the intelligibility. Tube V2 passes the amplified speech to a detector GI, supplying across a filter chain and for C a biasing potential for the exponential tube V3, whereby said bias has been detected in accordance with the input potential. Tube V3 has a high negative bias, while the detector is connected in such manner that at an increase of the speech potential, it furnishes an increasing counter potential to the fixed bias for tube Vs, so that for large amplitudes the degree of amplification becomes higher. In this way, the pure speech amplitudes become more distinctive from the smaller amplitudes of the residual disturbance.

What is claimed is:

1. A radio transmission system including a transmitter and a receiver, said transmitter having means for adding a disturbance potential to the speech potential so as to maintain secrecy of said speech, said means comprising a disturbance generator whose potential is superimposed upon said speech voltage by a microphone transformer which is connected to a grid of an electron discharge device, the plate circuit of said electron discharge device having its output circuit coupled by a series condenser, a shunted resistance and a translating device, said receiver having its input circuit elements arranged symmetrically and identical in number with respect to said transmitter.

2. A radio transmission system according to claim 1 characterized in that the phase distortion of said disturbance generator is maintained at a value less than five percent in the transmission range by selected values of said series condenser, shunted resistance and translating device.

3. A system according to claim 1 characterized in that the apparent resistance of the disturbance generator and the speech transformer is at a value which is low as compared with the grid cathode resistances of said electron discharge device.

4. A radio transmission system including a transmitter and a receiver, said transmitter having means for adding a disturbance potential to the speech potential so as to maintain secrecy of said speech, said means comprising a disturbance generator whose potential is superimposed upon said speech voltage, said speech voltage being coupled to the disturbance generator and a grid of an electron discharge device, the plate circuit of said device having its output coupled by a series condenser, a shunted resistance, and a translating device, said receiver having its input circuit elements arranged symmetrically and identical in number with respect to said transmitter.

5. A radio transmission system including a transmitter and a receiver, said transmitter having means for adding a disturbance potential to the speech potential so as to maintain secrecy of said speech, said means comprising a disturbance generator whose potential is superimposed upon said speech voltage by a microphone transformer which is connected to a grid of an electron discharge device, the plate circuit of said electron discharge device having its output circuit coupled by a series condenser, a shunted resistance and a translating device, said receiver comprising a controlled high frequency amplifier connected to a linear detector, the output of which is connected to circuit elements arranged symmetrically and identical in number with respect to said transmitter.

6. A radio transmission system including a transmitter and a receiver, said transmitter having means for adding a disturbance potential to the speech potential so as to maintain secrecy of said speech, said means comprising a disturbance generator whose potential is superimposed upon said speech voltage by a microphone transformer having two output windings, one of said output windings connected to the grid of a first electron discharge device, the other output winding of said microphone transformer connected to a grid circuit of a second electron discharge device having at least two grids, means for dynamical control of said speech potential, said means comprising a first and a second audio frequency transformer having their inputs connected to the respective plate circuits of said first and second electron discharge devices, a third electron discharge device having at least twogrids, the output of said first audio frequency transformer connected to a detector to rectify the output thereof, a filter including a condenser and a resistance connected to the output of said rectifier, said filter connected to the input grid of said third electron discharge device so that the detected potential from said rectifier increases the negative bias on said third electron discharge device as the speech potential increases in amplitude, means for coupling the plate circuit of said third electron discharge device to said disturbance generator, the combined potentials from said speech microphone and said disturbance generator being connected to the grid circuit of a fourth electron discharge device, the plate circuit of said fourth electron discharge device having its output circuit coupled by a series condenser, a shunted resistance and a translating device, said receiver having its input circuit elements arranged symmetrically and identical in number with respect to said transmitter.

PAUL KOTOWSKI. KURT DANNEHL. 

